The present invention relates generally an information storage device for use in a data processing system or the like and more particularly an information storage device which may permit the use of memory cells which would be rejected in the normal production process.
In general, an information storage device must include a set of completely operative memory cells so that any of the memory cells selected may execute the correct operation. With the remarkable recent advance of the semiconductor technology, there have been mass produced information storage devices of the type in which a large number of semiconductor memory cells are integrated on a single silicon chip. However the yield of the information storage devices with every memory cell completely operative is very low. In practice, even only one of the memory cells of the information storage device is rendered inoperative, an array of memory cells including an inoperative memory cell is also rendered inoperative. Therefore the information storage device of the type described is rather not economical.
An error-checking code may be used in some cases to correct information data retrieved from an information storage device including a small number of inoperative memory cells. However, this method is effective only when only a small number of bit cells in one word are inoperative. Therefore this method cannot be employed when the information storage device includes a relatively large number of memory elements including inoperative memory cells. In order to permit the use of the memory elements each including inoperative memory cells, there has been proposed to interconnect the memory cells by by-passing the inoperative memory cells. However, the inoperative arrays of memory cells must pass through an additional production step. In case of an element consisting of a large number of semiconductor memory cells, the cost will be inevitably increased.